1. Field of the Invention
The present invention relates to a drive control circuit of a vibration speaker having both a vibration function and a speaker function.
2. Description of the Related Art
A vibration speaker equipped with a vibration function and a speaker function is in practice use today. Since the vibration speaker is equipped with the both functions, it is expected that mobile devices (e.g., mobile phones, smartphones, portable game devices) into which the vibration speakers are incorporated be smaller in size and light in weight.
The vibration speaker, which is basically of the same configuration as a dynamic speaker, is provided with a voice coil, a magnetic circuit, and diaphragm. The force produced by the electricity flowing through the voice coil and the magnetism by the magnetic circuit is exerted on the magnetic circuit and the vibration plate. Though the magnetic circuit weighs a certain weight, the diaphragm is designed to be lightweight. Where a low-frequency signal is inputted to the voice coil, the magnetic circuit vibrates efficiently and the vibration function can be achieved fully. Where a high-frequency signal is inputted thereto, the magnetic circuit barely vibrates due to the weight itself. However, the diaphragm vibrates efficiently and therefore the speaker function can be achieved fully.
In a vibration mode where the vibration function of the vibration speaker is achieved, the vibration speaker is preferably driven at a frequency as close to its eigen frequency as possible (hereinafter, this eigen frequency will be referred to as “resonance frequency” also). The vibration speaker generates the most powerful vibration when the resonance frequency thereof agrees with the drive frequency.
Since the eigen frequency of the vibration speaker in the vibration mode is determined mainly by the magnetic circuit, the eigen frequency varies from one product to another. When the magnetic circuit is suspended by a frame through the spring, the eigen-frequency also varies according to the spring const.
Thus, in the conventional method where a fixed drive frequency is set to all drive control circuits for the vibration speakers, there are drive control circuits with a significant disagreement between the eigen frequency of the vibration speaker and the drive frequency thereof, thereby causing a drop in the yield. Also, even though the eigen frequency of the vibration speaker and the drive frequency thereof agree at first, there are cases where they deviate from each other with time and the vibration gets weaker.